IconManagerService::formatSearchTerms() throws error if 'search_terms' is missing

Problem/Motivation

The formatSearchTerms() method in IconManagerService assumes that the search_terms property exists and is always an array. However, if an icon is missing search_terms (or it is NULL), the method will trigger a fatal TypeError:

TypeError: array_push(): Argument #1 ($array) must be of type array, null given in array_push()

Steps to reproduce

• Install and enable the Fontawesome Iconpicker Widget module.
• Use the service fontawesome_iconpicker_widget.icon_manager in custom code (for example, in a field widget plugin).
  Example code calling:
  $term_list = \Drupal::service('fontawesome_iconpicker_widget.icon_manager')->getFormattedTermList();.
• Ensure at least one FontAwesome icon does not have search_terms populated.
• Render the form using your widget (or otherwise trigger the code path).
• Observe a TypeError is thrown:
  TypeError: array_push(): Argument #1 ($array) must be of type array, null given

Proposed resolution

Update formatSearchTerms() to defensively check that $properties['search_terms'] is an array before trying to array_push() into it. If search_terms is missing or NULL, initialize it as an empty array before pushing. This makes the method resilient against incomplete or custom icon metadata.

Remaining tasks

• Review the attached patch.
• If the change is deemed useful by the maintainers I can create a merge request.

User interface changes

API changes

Data model changes